#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <utility>
#include <vector>
using namespace std;

#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int((x).size()))
#define bit(x) (1 << (x))
#define cnt1(x) (__builtin_popcount(x))

template<class T>
inline void chkmax(T& x, T y) {
    if (x < y) x = y;
}

template<class T>
inline void chkmin(T& x, T y) {
    if (y < x) x = y;
}

typedef long long LL;
typedef double DB;
typedef pair<int, int> PII;
typedef vector<int> VI;

int main() {
    
    int tc, n, m, p, q, val;
    
    for (scanf("%d", &tc); tc--; ) {
        scanf("%d%d%d%d", &n, &m, &p, &q);
        val = p * n;
        chkmin(val, q * (n / m) + p * (n % m));
        chkmin(val, q * (n / m + 1));
        printf("%d\n", val);
    }
    return 0;
}